1. Field of the Invention
The present invention relates to voltage-nonlinear resistors, the method for making the same, and varistors using the same.
2. Description of the Related Art
With miniaturization of circuits and shift of reference frequencies towards higher regions, miniaturized electronic components, which can work in higher frequencies, are required. Moreover, electronic components which can work in lower voltages are required, as the drive voltages of the circuits tend to decrease. Varistors as abnormal-voltage absorbing elements also must have such requirements.
SiC-based, ZnO-based and SrTiO3-based voltage-nonlinear resistors for constituting varistors are known. ZnO-based and SrTiO3-based voltage-nonlinear resistors are used in monolithic varistors having driving voltages of 3.5 V or more.
When varistors are used as noise-absorbing elements of signal circuits and the like in higher frequencies, the varistors must have reduced electrostatic capacitance. Moreover, the varistor voltage must be low in order to use them at a lower drive voltage.
Conventional ZnO-based varistors have apparent relative dielectric constants xcex5r of 200 or more. The apparent relative dielectric constants of SrTiO3-based varistors are on the order of several thousands to several tens of thousands and are higher than that of the ZnO-based varistors. Thus, in order to reduce electrostatic capacitance in the varistor, the electrode area of the varistor must be significantly reduced or the thickness of the voltage-nonlinear resistor must be increased so as to increase the distance between the varistor electrodes. A reduced electrode area, however, causes a decreased surge current capacity, whereas a decreased varistor voltage causes increased electrostatic capacitance. Accordingly, it is difficult to simultaneously achieve a low voltage and low capacitance.
Since SiC-based varistors have low apparent relative dielectric constants ∈r, low electrostatic capacitance can be readily obtained. The SiC-based varistors, however, exhibit lower voltage-nonlinearity coefficients (xcex1) compared to other types of varistors. That is, the coefficients of the SiC-based varistors are at most 8, whereas the coefficients of the ZnO-based and SrTiO3-based varistors are several tens.
Accordingly, it is an object of the present invention to provide a voltage-nonlinear resistor having small electrostatic capacitance, a high voltage-nonlinearity coefficient ox and a high varistor voltage, to provide a method for making the voltage-nonlinear resistor, and to provide a varistor using the voltage-nonlinear resistor.
According to an aspect of the present invention, a voltage-nonlinear resistor comprises SiC particles as a primary component, the SiC particles being doped with a dopant and containing at least one of elemental Al and B. Preferably, the at least one Al or B element is coordinated on the surfaces of the SiC particles doped with the dopant. Preferably, the surfaces of the SiC particles doped with the dopant are oxidized. The total content of Al and B is in a range of preferably about 0.01 parts by weight to 100 parts by weight and more preferably about 0.5 parts by weight to 50 parts by weight with respect to 100 parts by weight of the SiC particles doped with the dopant. Preferably, the dopant is at least one of elemental N and P. The total content of the dopant is preferably in a range of about 30 ppm to 10,000 ppm. The SiC doped with the dopant is thereby an n-type semiconductor. Preferably, the SiC has a xcex2-type crystal system.
The resulting voltage-nonlinear resistor exhibits a low apparent relative dielectric constant ∈r, a voltage-nonlineanity coefficient xcex1 and a low varistor voltage.
According to another aspect of the present invention, a method for making a voltage-nonlinear resistor comprises: mixing SiC doped with a dopant and at least one of Al and B to prepare a powdered mixture, heat-treating the powdered mixture in an oxidizing atmosphere to form a metal oxide crystal phase comprising at least one of Al2O3 and Al6Si2O13 and a SiO2 crystal phase. Preferably, the heat treatment is performed at a temperature of about 1,000xc2x0 C. to 1,600xc2x0 C. This method facilitates coordination of Al and B on the surfaces of the SiC particles and oxidation of the surfaces of the SiC particles.
According to another aspect of the present invention, a varistor comprises the above voltage-nonlinear resistor and varistor electrodes.
The varistors comprising the voltage-nonlinear resistor has superior varistor characteristics.